Infrared radiation emitted from a planet contains information about the chemical composition and vertical temperature profile of its atmosphere1,2,3. If upper layers are cooler than lower layers, molecular gases will produce absorption features in the planetary thermal spectrum4,5. Conversely, if there is a stratosphere—where temperature increases with altitude—these molecular features will be observed in emission6,7,8. It has been suggested that stratospheres could form in highly irradiated exoplanets9,10, but the extent to which this occurs is unresolved both theoretically11,12 and observationally3,13,14,15. A previous claim for the presence of a stratosphere14 remains open to question, owing to the challenges posed by the highly variable host star and the low spectral resolution of the measurements3. Here we report a near-infrared thermal spectrum for the ultrahot gas giant WASP-121b, which has an equilibrium temperature of approximately 2,500 kelvin. Water is resolved in emission, providing a detection of an exoplanet stratosphere at 5σ confidence. These observations imply that a substantial fraction of incident stellar radiation is retained at high altitudes in the atmosphere, possibly by absorbing chemical species such as gaseous vanadium oxide and titanium oxide.
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Madhusudhan, N. & Seager, S. A temperature and abundance retrieval method for exoplanet atmospheres. Astrophys. J. 707, 24–39 (2009)
Line, M. R. et al. A systematic retrieval analysis of secondary eclipse spectra. I. A comparison of atmospheric retrieval techniques. Astrophys. J. 775, 137 (2013)
Line, M. R. et al. No thermal inversion and a solar water abundance for the hot Jupiter HD 209458b from HST/WFC3 spectroscopy. Astrophys. J. 152, 203 (2016)
Stevenson, K. B. et al. Thermal structure of an exoplanet atmosphere from phase-resolved emission spectroscopy. Science 346, 838–841 (2014)
Beatty, T. G. et al. Evidence for atmospheric cold-trap process in the non-inverted emission spectrum of Kepler-13Ab using HST/WFC3. Preprint at https://arxiv.org/abs/1612.06409 (2016)
Gillett, F. C., Low, F. J. & Stein, W. A. The 2.8–14-micron spectrum of Jupiter. Astrophys. J. 157, 925–934 (1969)
Ridgway, S. T. Jupiter: identification of ethane and acetylene. Astrophys. J. 187, L41–L43 (1974)
Wallace, L., Prather, M. & Belton, M. J. S. The thermal structure of the atmosphere of Jupiter. Astrophys. J. 193, 481–493 (1974)
Fortney, J. J., Lodders, K., Marley, M. S. & Freedman, R. S. A unified theory for the atmospheres of the hot and very hot Jupiters: two classes of irradiated atmospheres. Astrophys. J. 678, 1419–1435 (2008)
Hubeny, I., Burrows, A. & Sudarsky, D. A possible bifurcation in atmospheres of strongly irradiated stars and planets. Astrophys. J. 594, 1011–1018 (2003)
Burrows, A., Budaj, J. & Hubeny, I. Theoretical spectra and light curves of close-in extrasolar giant planets and comparison with data. Astrophys. J. 678, 1436–1457 (2008)
Spiegel, D. S., Silverio, K. & Burrows, A. Can TiO explain thermal inversions in the upper atmospheres of irradiated giant planets? Astrophys. J. 699, 1487–1500 (2009)
Knutson, H. A., Charbonneau, D., Allen, L. A., Burrows, A. & Megeath, S. T. The 3.6–8.0μm broadband emission spectrum of HD209458b: evidence for an atmospheric temperature inversion. Astrophys. J. 673, 526–531 (2008)
Haynes, K., Mandell, A. M., Madhusudhan, N., Deming, D. & Knutson, H. Spectroscopic evidence for a temperature inversion in the dayside atmosphere of hot Jupiter WASP-33b. Astrophys. J. 806, 146 (2015)
Wong, I. et al. 3.6 and 4.5 μm Spitzer phase curves of the highly irradiated hot Jupiters WASP-19b and HAT-P-7b. Astrophys. J. 823, 122 (2016)
Cartier, K. M. S. et al. Near-infrared emission spectrum of WASP-103b using Hubble Space Telescope/Wide Field Camera 3. Astron. J. 153, 34 (2017)
Crouzet, N., McCullough, P. R., Deming, D. & Madhusudhan, N. Water vapor in the spectrum of the extrasolar planet HD 189733b. II. The eclipse. Astrophys. J. 795, 166 (2014)
Wilkins, A. N. et al. The emergent 1.1–1.7 μm spectrum of the exoplanet COROT-2b as measured using the Hubble Space Telescope. Astrophys. J. 783, 113 (2014)
Evans, T. M. et al. Detection of H2O and evidence for TiO/VO in an ultra-hot exoplanet atmosphere. Astrophys. J. 822, L4 (2016)
Delrez, L. et al. WASP-121 b: a hot Jupiter close to tidal disruption transiting an active F star. Mon. Not. R. Astron. Soc. 458, 4025–4043 (2016)
Sharp, C. M. & Burrows, A. Atomic and molecular opacities for brown dwarf and giant planet atmospheres. Astrophys. J. Suppl. Ser. 168, 140–166 (2007)
Wakeford, H. R. & Sing, D. K. Transmission spectral properties of clouds for hot Jupiter exoplanets. Astron. Astrophys. 573, A122 (2015)
Guillot, T. On the radiative equilibrium of irradiated planetary atmospheres. Astron. Astrophys. 520, A27 (2010)
Marley, M. S. & McKay, C. P. Thermal structure of Uranus’ atmosphere. Icarus 138, 268–286 (1999)
Hickey, M. P., Walterscheid, R. L. & Schubert, G. Gravity wave heating and cooling in Jupiter’s thermosphere. Icarus 148, 266–281 (2000)
Matcheva, K. I. & Strobel, D. F. Heating of Jupiter’s thermosphere by dissipation of gravity waves due to molecular viscosity and heat conduction. Icarus 140, 328–340 (1999)
Heng, K., Hayek, W., Pont, F. & Sing, D. K. On the effects of clouds and hazes in the atmospheres of hot Jupiters: semi-analytical temperature-pressure profiles. Mon. Not. R. Astron. Soc. 420, 20–36 (2012)
Lodders, K. Titanium and vanadium chemistry in low-mass dwarf stars. Astrophys. J. 577, 974–985 (2002)
Sing, D. K. et al. A continuum from clear to cloudy hot-Jupiter exoplanets without primordial water depletion. Nature 529, 59–62 (2016)
Kirkpatrick, J. D. et al. Discoveries from a near-infrared proper motion survey using multi-epoch Two Micron All-Sky Survey data. Astrophys. J. Suppl. Ser. 190, 100–146 (2010)
Hauschildt, P. H., Allard, F. & Baron, E. The NextGen model atmosphere grid for 3,000 ≤ Teff ≤ 10,000 K. Astrophys. J. 512, 377–385 (1999)
Deming, D. et al. Infrared transmission spectroscopy of the exoplanets HD 209458b and XO-1b using the Wide Field Camera 3 on the Hubble Space Telescope. Astrophys. J. 774, 95 (2013)
Wakeford, H. R. et al. HST PanCET program: a cloudy atmosphere for the promising JWST target WASP-101b. Astrophys. J. 835, L12 (2017)
Mandel, K. & Agol, E. Analytic light curves for planetary transit searches. Astrophys. J. 580, L171–L175 (2002)
Foreman-Mackey, D., Hogg, D. W., Lang, D. & Goodman, J. emcee: the MCMC hammer. Publ. Astron. Soc. Pacif. 125, 306–312 (2013)
Jones, E. et al. SciPy: Open source scientific tools for Python. http://www.scipy.org (2001)
Gelman, A. & Rubin, D. B. Inference from iterative simulation using multiple sequences. Stat. Sci. 7, 457–472 (1992)
Lewis, N. K. et al. Orbital phase variations of the eccentric giant planet HAT-P-2b. Astrophys. J. 766, 95 (2013)
Deming, D. et al. Spitzer secondary eclipses of the dense, modestly-irradiated, giant exoplanet HAT-P-20b using pixel-level decorrelation. 805, 132 (2015)
Amundsen, D. S. et al. Accuracy tests of radiation schemes used in hot Jupiter global circulation models. Astron. Astrophys. 564, A59 (2014)
Amundsen, D. S., Tremblin, P., Manners, J., Baraffe, I. & Mayne, N. J. Treatment of overlapping gaseous absorption bands with the correlated-k method in hot Jupiter and brown dwarf atmosphere models. Astron. Astrophys. 598, A97 (2017)
Drummond, B. et al. The effects of consistent chemical kinetics calculations on the pressure-temperature profiles and emission spectra of hot Jupiters. Astron. Astrophys. 594, A69 (2016)
Tremblin, P. et al. Fingering convection and cloudless models for cool brown dwarf atmospheres. Astrophys. J. 804, L17 (2015)
Tremblin, P. et al. Cloudless atmospheres for L/T dwarfs and extrasolar giant planets. Astrophys. J. 817, L19 (2016)
Wakeford, H. R. et al. HAT-P-26b: a Neptune-mass exoplanet with a well-constrained heavy element abundance. Science 356, 628–631 (2017)
Eastman, J., Gaudi, B. S. & Agol, E. EXOFAST: a fast exoplanetary fitting suite in IDL. Publ. Astron. Soc. Pacif. 125, 83–112 (2013)
Kreidberg, L. et al. A precise water abundance measurement for the hot Jupiter WASP-43b. Astrophys. J. 793, L27 (2014)
Stevenson, K. B. et al. Spitzer phase curve constraints for WASP-43b at 3.6 and 4.5 μm. Astron. J. 153, 68 (2017)
Rothman, L. S. et al. The HITRAN 2008 molecular spectroscopic database. J. Quant. Spectrosc. Radiat. Transf. 110, 533–572 (2009)
Tennyson, J. et al. The ExoMol database: molecular line lists for exoplanet and other hot atmospheres. J. Mol. Spectrosc. 327, 73–94 (2016)
Eastman, J., Siverd, R. & Gaudi, B. Scott Achieving better than 1 minute accuracy in the heliocentric and barycentric Julian Dates. Publ. Astron. Soc. Pacif. 122, 935–946 (2010)
This work is based on observations with the NASA/ESA HST, obtained at the Space Telescope Science Institute (STScI) operated by AURA, Inc. This work is also based in part on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement no. 336792 and is supported by the ERC Horizon 2020 research and innovation programme (grant agreement no. 724427). Support for this work was provided by NASA through grants under the HST-GO-14767 “Panchromatic Comparative Exoplanetary Treasury (PanCET)” programme from the STScI. J.G. acknowledges support from a Leverhulme Trust Research Project Grant. H.R.W acknowledges support from the NASA Postdoctoral Program, administered by Universities Space Research Association through a contract with NASA. M.S.M. acknowledges support from the NASA Exoplanets Research Program. J.K.B. acknowledges support from a Royal Astronomical Society Fellowship. D.E. and V.B. acknowledge the financial support of the National Centre for Competence in Research “PlanetS” supported by the Swiss National Science Foundation (SNSF). A.L.E. acknowledges support from CNES and the French Agence Nationale de la Recherche (ANR), under programme ANR-12-BS05-0012 “Exo-Atmos”. J.S.-F. acknowledges support from the Spanish MINECO through grant AYA2014-54348-C3-2-R. G.W.H. acknowledges support from NASA, NSF, Tennessee State University, and the State of Tennessee through its Centers of Excellence programme. L.B.-J. and P.L. acknowledge support from CNES (France) under project PACES. P.T. and D.S.A. acknowledge funding from the European Research Council under the European Union Seventh Framework Program: grant 247060-PEPS.
The authors declare no competing financial interests.
Reviewer Information Nature thanks K. Heng and the other anonymous reviewer(s) for their contribution to the peer review of this work.
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Extended data figures and tables
a–d, Raw normalized light curves for each of the spectroscopic channels with photon-noise 1σ error bars (orange and green circles), and best-fit eclipse signals multiplied by linear time trends (grey lines). Vertical offsets have been applied for visual clarity. Labels indicate central wavelengths for each channel. e–h, Model residuals after removing the best-fit eclipse signal and linear time trend for each of the spectroscopic light curves in a–d, respectively, with photon-noise 1σ error bars (orange and green circles) and stochastic components of the best-fit Gaussian process models (black lines). Note that for most channels, the correlations remaining in the residuals after accounting for a linear time trend are minimal.
Extended Data Figure 2 Model thermal spectrum for WASP-121b broken down by emission source and absorption cross-sections of important molecules.
a, Similar to Fig. 2, showing the HST measurements of the WASP-121b thermal spectrum (brown circles; error bars, 1σ uncertainties). Red line shows the best-fit model (H2O and VO) obtained from the retrieval analysis. Other coloured lines show model thermal spectra generated using the same T–P profile as for the best-fit model but with the opacity due to each molecule turned off one-by-one: VO off (blue line) and H2O off (green line). b, Absorption cross-sections for plausible gas-phase absorbers (colour coded, key at top right) across the WFC3 bandpass. Text labels in the key give the rescaling factors that have been applied to each cross-section to fit them on a single vertical axis, with the exception of the H2O cross-section which has not been rescaled. Note that the VO cross-section has been rescaled by a factor of 0.03, which is consistent with the abundance of VO relative to H2O inferred for the best-fit model (Extended Data Table 4).
a, A sequence of T–P profiles with and without stratospheres computed using the analytic parameterization of ref. 23. The red line corresponds to a model with a strong stratosphere obtained from the MCMC retrieval analysis and the green line shows a model with a weaker stratosphere. The light blue and dark blue lines show, respectively, models with a decreasing temperature profile and a strongly-decreasing temperature profile. The yellow line indicates the best-fit isothermal model with a temperature of 2,700 K. b, Corresponding contribution functions averaged over the WFC3 G141 bandpass, indicating the approximate pressures probed at these wavelengths. c, Corresponding thermal spectra, with H2O and VO bands seen in emission for the models with stratospheres (red and green lines) and in absorption for the models with decreasing temperature profiles (light blue and dark blue lines). The isothermal model has a featureless blackbody spectrum (yellow line).
a, Similar to Fig. 2, but showing self-consistent models in radiative equilibrium: median H2O and VO abundances obtained from the retrieval analysis (light blue line); H2O abundance set to median value and VO abundance set to the lower value of the 68% credible range (green line); solar metallicity, but without VO and TiO, and with an arbitrary absorber with absorption cross-section κ = 0.02 cm2 g−1 (pink line); and 15× solar metallicity, but without VO and TiO, and with an arbitrary absorber with κ = 0.06 cm2 g−1 (dark blue line). For the last two models, the arbitrary absorber has grey opacity across the 0.43–1 μm wavelength range and is assumed to be distributed uniformly throughout the atmosphere. The best-fit isothermal model with temperature 2,700 K is also shown (yellow line). Brown circles with error bars, HST data with 1σ uncertainties: inset, HST data on a magnified scale. b, Corresponding T–P profiles, all of which have a stratosphere across the 10−2 bar to 10−5 bar pressure range. Thick lines indicate the main pressure levels probed across the WFC3 G141 bandpass.
Panels along the diagonal show the marginalized posterior distributions for each parameter of the thermal emission model (κIR, γ1, β, H2O abundance, VO abundance), shown left to right in the columns, and top to bottom in the rows. Solid orange lines show the posterior medians and dashed orange lines show the ranges either side encompassing ±34% of the samples. Panels below the diagonal show the posterior distribution as a function of each parameter pair with increasing probability from light to dark shades. As γ1 controls the optical opacity, there is a strong anti-correlation with the radiation efficiency factor β, where lower radiation efficiency values are compensated by higher optical opacities and vice versa. There is also a strong degeneracy between the abundances of each molecule (H2O, VO) and the parameter κIR which controls the overall atmospheric opacity. Higher abundances and higher values of κIR result in stratospheres at lower pressures and vice versa.
a, Black data points show the measured thermal emission data for WASP-43b taken from ref. 48, composed of HST/WFC3 and Spitzer/IRAC observations. Vertical error bars give 1σ uncertainties and horizontal error bars give photometric bandpasses. The red line shows the best-fit spectrum obtained from an ATMO retrieval analysis, with shading indicating the regions encompassing 68%, 95% and 99.7% of the MCMC samples. The inset shows a magnified view of the HST data. The retrieved volume mixing ratio (VMR) for H2O is in good agreement with the published value obtained using the CHIMERA retrieval code2,3 and the ATMO retrieval achieves a better fit to the data as measured by the minimum χ2. b, Best-fit T–P profiles retrieved with ATMO (solid blue line) and CHIMERA (solid green line). The two analyses agree to within the 68% credible ranges (blue and green dashed lines).
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Evans, T., Sing, D., Kataria, T. et al. An ultrahot gas-giant exoplanet with a stratosphere. Nature 548, 58–61 (2017). https://doi.org/10.1038/nature23266
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